As oil and gas reserves become scarcer or depleted, methods for more efficient production have to be developed.
In recent years horizontal drilling has proved to enhance greatly the rate of production from wells producing in tight or depleted formation. Tight formations typically are hydrocarbon-bearing formations with poor permeability, such as the Austin Chalk in the United States and the Danian Chalk in the Danish Sector of the North Sea.
In these tight formations oil production rates have dropped rapidly when conventional wells have been drilled. This is due to the small section of producing formation open to the well bore.
However when the well bore has been drilled horizontally through the oil producing zones, the producing section of the hole is greatly extended resulting in dramatic increases in production. This has also proved to be effective in depleted formations which have been produced for some years and have dropped in production output.
However, horizontal drilling has many inherent difficulties. In broad terms the difficulties include the following factors:
(i) the rotational torque requirement of the drillstring rises rapidly with increasing hole angle (angular displacement from vertical) and length of the horizontal section, PA1 (ii) the weight of the drillstring in the vertical section of the hole must push the drillpipe along the horizontal section thereby increasing the fatigue stresses in the drillpipe located on the bend between the two sections, PA1 (iii) performance of the drillbit is reduced due to both (i) and (ii) above as difficulties in applying weight and torque affect the ROP ("rate of progress" in deepening/lengthening of the well).
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